This invention relates to injectors for injection of pre-polarised fluid into patients for magnetic resonance (MR) measurements.
As is well known, in MR imaging or spectroscopy measurements, a magnetic field is applied to a region of interest of the subject which causes nuclei with magnetic moments to align along the direction of the magnetic field, so that the region of interest acquires a net magnetisation parallel to the field. Application of an r.f. electromagnetic pulse of an appropriate frequency (the Larmor frequency) by way of a transmit coil in a direction orthogonal to the direction of the magnetic field excites the nuclei to resonance, thereby orienting the net magnetisation vector out of alignment with the main magnetic field. The subsequent r.f. relaxation signal which can be detected by a receive coil, generated by the nuclei as their return to their equilibrium condition in alignment with the main magnetic field, enables information about the material in question to be ascertained. To produce an image, coils are provided to vary the strength, but not the direction of the main magnetic field, in order to spatially encode the relaxation signals.
It is sometimes desired to produce an image e.g. of tissue of a patient, which gives prominence to vessels e.g. blood vessels, and the flow of blood is utilised to achieve this. In one method, the so-called "time-of-flight" method, a relatively short repetition time between successive r.f. excitation pulses is selected. Each r.f. pulse flips the longitudinal component of the net magnetisation out of alignment with the main magnetic field, and the longitudinal component then starts to recover to its original value. Since the repetition time is short, the longitudinal component does not have time to recover fully before the next and subsequent r.f. pulses, and achieves a steady state value considerably less than the undisturbed value. As far as blood entering a vessel is concerned, however, this has not been subjected to any r.f. pulses because these are specific to the region (usually a slice) being imaged. The longitudinal component of magnetisation of the moving blood is then undiminished by the saturation effect described for stationary tissue. Hence a larger signal is produced as the previously unexcited blood receives an excitation pulse within the imaged slice. As an alternative to the so-called time-of-flight methods described, phase contrasts methods have been used. A reference image of stationary nuclei is subtracted from an image of stationary nuclei and nuclei moving at a particular velocity, obtained by applying a gradient and inducing a phase shift of values appropriate to that velocity.
However, in both of these methods, the signal-to-noise ratio of the MR signal is improved if the main magnetic field is increased. This can be done, but implies the use of a larger, more expensive magnet.
A proposal has been made (U.S. Pat. No. 5,479,925, U.S. Pat. No. 5,626,137, U.S. Pat. No. 5,611,340, U.S. Pat. No. 5,617,859) to inject a pre-polarised fluid into a catheter inserted into the region of interest of a patient prior to MR imaging. A separate high power magnet has to be provided to pre-polarise the fluid, but this is much cheaper than making a corresponding increase in the strength of the main MR imaging magnet, since the latter has to provide a very homogeneous field, whereas the former does not. The pre-polarised fluid e.g. saline then produces a significantly increased signal in the vessels into which the catheter feeds. Clearly, the transfer and delivery must be achieved on a time scale that is short or comparable to the T1 relaxation time (relating to the time of decay of longitudinal magnetisation of the nuclei) or otherwise the stored magnetism will decay so much that little net signal enhancement will be detected.
The pre-polarising magnet must be housed at some distance from the main imaging magnet, or stray fields from the pre-polarising magnet will affect the homogeneity of the field of the imaging magnet.
There are two problems. Firstly, the rate of flow down the tube is limited by safety considerations. A faster flow would risk damaging any blood vessels. A flow of 5-9 ml/sec is the maximum feasible for veins, and this places a limit on the minimum transmit time from the large pre-polarising field to the MR scanner used to image the patient. Secondly, all the fluid in the tube that is between the pre-polarising field and the MR scanner must be pushed into the blood vessel before the pre-polarised material arrives.